This invention relates generally to Scanning Electron Microscope (SEM)-based scanning of specimens and, more particularly, to a SEM based method of scanning of semiconductor for defect inspection, and to a SEM-based method of scanning photo mask for defect inspection.
Scanning methods of different kinds are used in applications that involve inspection of very small structures of objects in great detail. Some of these applications include defect review and inspection of specimens such as very large scale integrated (VLSI) circuits, wafers, or other articles, critical dimensioning of features in these specimens as well as design and process verification of the specimens. In such applications, scanning electron microscopes (SEM) are typically employed for scanning the specimens.
Scanning electron microscopes use an electron beam to scan the specimen. A SEM makes use of the wave nature of electrons to produce images with high resolution. A SEM operates by generating a beam of electrons called a particle beam. The particle beam is collimated by condenser lenses of the SEM and focused on the surface of the specimen through an objective lens system. The focused particle beam is deflected with the help of a deflection system of the SEM. The deflection system helps in moving the focused particle beam across the surface of the specimen in a particular direction for purposes of scanning. The deflected particle beam collides with specimen and generates secondary electrons (SE) and back-scattered electrons (BSE). The SE and BSE are then captured by a detector system of the SEM to produce an image of the specimen. The produced image is useful in inspecting the minute structures of the specimen under examination in great detail.
According to conventional SEM scanning methods for wafer inspection, the particle beam is first focused on the wafer and is then deflected either in parallel or perpendicular direction with respect to the die orientation of the wafer. However, such a parallel or perpendicular direction of the deflected particle beam does not help in overcoming the edge effect or aliasing effect which commonly occurs when scanning the patterned structures on a semiconductor wafer. Aliasing effect refers to the situation where the image lines appear to have jagged edges. During wafer scanning, aliasing may be caused by a slight misalignment between the die orientation and the particle beam scanning direction, thus resulting in an apparent detection of non-uniformity along the edges of the patterned structure on the surface of the wafer by a detector system. This leads to an otherwise satisfactory wafer being categorized as faulty.
It is therefore a goal of the invention to provide a method in reducing the edge effect or aliasing effect in wafer inspections.
The aforementioned and related drawbacks associated with conventional SEM and methods of scanning specimens using a SEM are substantially reduced or eliminated by the present invention. The present invention is directed to a SEM, which scans the surface of a semiconductor wafer by performing the following steps: generating a particle beam from a particle beam emitter; and scanning the surface of the semiconductor wafer by deflecting the particle beam to a direction relative to the die orientation, such that the particle beam traverses a direction that is neither parallel nor perpendicular to the die orientation of the semiconductor wafer. According to the present invention, the particle beam is deflected onto the surface of the semiconductor so as to scan the surface of the semiconductor wafer at an angle within the preferred ranges of (1xc2x0 and 89xc2x0) or (91xc2x0 and 179xc2x0) or (xe2x88x9289xc2x0 and xe2x88x921xc2x0) or (xe2x88x92179xc2x0 and xe2x88x9291xc2x0) with respect to the die orientation. According to the present invention, the SEM comprises: a particle beam emitter for emitting a particle beam in a SEM; and a deflection unit which is to scan the surface of the semiconductor wafer by deflecting the particle beam at an angle relative to the die orientation of the semiconductor wafer, such that the particle beam traverses a direction that is neither parallel nor perpendicular to the orientation of the die on the semiconductor wafer.
An advantage of the present invention is that the non-parallel and non-perpendicular scanning angles reduce aliasing effects.
Another advantage of the present invention is that the non-parallel and non-perpendicular scanning angles can be easily implemented with current scanning electron microscopes.